Secondary batteries such as lithium secondary batteries and nickel-hydrogen batteries are preferably used as so-called portable power supplies in personal computers, mobile devices, etc., or as vehicle-driving power supplies. In particular, lithium secondary batteries that are lightweight and have high energy densities have become increasingly important as high output power supplies for use on vehicles such as electric automobiles, hybrid mobiles and the like (e.g. as power sources to drive motors connected to drive wheels of vehicles).
Typical examples of such secondary batteries include a hermetic battery (sealed secondary battery) obtainable by sealing an electrode unit having a positive electrode and a negative electrode along with an electrolyte in a battery case. When charging this type of battery, in the presence of a defect in the battery or when a malfunction is caused by a charger failure, the electrode unit may be supplied with a higher current than usual and fall in an overcharged state. Upon the occurrence of such an overcharged state, the battery reaction rapidly proceeds and the electrolyte solution in the battery undergoes decomposition at an accelerating rate to produce a large amount of gas. This may increase the internal pressure (gas pressure) of the battery case and cause deformation of the battery case.
To prevent such undesirable effects caused by overcharging of a sealed secondary battery, it has been suggested to arrange a current cutoff mechanism in a conductive path in the battery, with the mechanism comprising a member (so-called “current breaking valve”) that deforms when a certain minimum pressure is applied thereto. In a sealed secondary battery comprising such a current cutoff mechanism, when overcharging causes an internal pressure rise, the current breaking valve deforms to cutoff the conductive path in the battery. Technical literatures related to sealed secondary batteries comprising current cutoff mechanisms include Patent Documents 1 and 2.